In wireless transmission systems, diversity is a technique that provides a number of independent signals to reduce signal distortion caused by fading. For example, the diversity is achieved through a number of receive antennas in an uplink communication, and the same is achieved through multiple transmit antennas in a downlink communication.
V. Tarokh has proposed a method for achieving diversity using space-time codes, which can achieve diversity and coding gain through trellis encoding without increasing the bandwidth, and can increase system performance gain by efficiently taking advantage of error correcting codes and diversity transmission.
The method is described in an article entitled “Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction,” IEEE Transactions On Information Theory, Vol. 44, No. 2, P. 744 to 795, March 1998.
This prior art has proposed a method for achieving diversity gain in multiple transmit antennas using the same bandwidth as a single transmit antenna (i.e., without increasing the bandwidth) in a narrow-band fast mobile communication system. This prior art has established general rules, which can incorporate conventional diversity methods, and verified the general rules through a method using trellis coding, and also proposed a method for achieving diversity in multiple transmit antennas according to the general rules.
This prior art has suggested rules and examples of a type of encoding in which signal points, which are resources available in narrow bands, can be suitably associated with a number of transmit antennas, thereby suggesting the possibility that various transmission diversity methods can be provided, and theoretical background supporting the possibility. However, this prior art has suggested only the possibility that transmission diversity can be achieved using space-time codes that are constructed with dense signal constellations under the assumption of narrow band communication, and theoretical background supporting the possibility.
In addition, V. Tarokh and S. M. Alamouti have proposed a method using Space-Time Block Codes (STBC), which can achieve transmission diversity more easily than the method using Space-Time Trellis Codes (STTC). However, this method has problems in that it is difficult to obtain additional coding gain and it inevitably increases the bandwidth when more than 2 transmit antennas is used.
Also, studies have been carried out on a method for achieving transmission diversity in Code Division Multiple Access (CDMA) mobile communication using space-time codes. The simplest method for achieving transmission diversity in the CDMA mobile communication is to transmit signals to users after spreading the signals using spreading codes in respective transmit antennas. However, since a single spreading code must be assigned to each of the transmit antennas of the users, this method has a disadvantage in that the number of supportable users is reduced by a factor of the number of transmit antennas when orthogonal spreading codes are used or when the number of spreading codes is limited.
For example, under the assumption that M transmit antennas are provided for each user, it is possible to achieve M-fold diversity without using space-time codes when M spreading codes are assigned to each user. However, this method has a problem in that, when M spreading codes are assigned to each user and the total number of codes is limited, the number of supportable users is M−1 times smaller than that when only one transmit antenna is used.
One method to overcome this problem is to achieve transmission diversity in the CDMA mobile communication using Alamouti's codes or other space-time block codes.
The conventional method of achieving transmission diversity using space-time codes in CDMA transmission is associated with a method that uses spreading code sequences in CDMA wireless transmission, and more particularly with a method that transmits encoded messages in downlink for more secure transmission using multiple transmit antennas in the transmitter under a fading environment.
In this method, data symbol sets are transmitted to the users of one or more user groups, and a set of spreading code sequences, which is called a “code group”, is assigned to each of the user groups. Each of the data symbol sets is transmitted in the form of two or more signal sequences to the users of a given user group. Each of the signal sequences is transmitted through a corresponding one of two or more antennas. Here, each signal sequence is a linear combination of spreading code sequences belonging to the corresponding code group. In the linear combination of spreading code sequences, each spreading code sequence has a scalar coefficient, which is a linear combination of pertinent data symbols (for example, data symbols to be transmitted to users in a given user group) or of complex conjugates of pertinent data symbols.
Specifically, one method for achieving transmission diversity using space-time codes in CDMA transmission was disclosed in U.S. Pat. No. 6,452,916 (Sep. 17, 2002), entitled “Space-time spreading method of CDMA wireless communication”.
This patent provides a method that can accommodate the same maximum number of users as when one antenna is used with a limited total of spreading codes. Since this method uses the spreading code domain, assigned to other users of the user group, rather than the time domain, it can be considered a method employing the conventional space-time block codes. Although it is possible to accommodate the same number of users while achieving transmission diversity without increasing the total number of spreading codes, the method of U.S. Pat. No. 6,452,916 is disadvantageous in that a mobile station must know spreading codes of the other users in each group.
One conventional method for achieving transmission diversity in CDMA mobile communication will now be described in detail with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram of a conventional diversity transmitter, and FIG. 2 is a block diagram of a conventional diversity receiver.
As shown in FIG. 1, the conventional diversity transmitter includes a data source 110, a plurality of multipliers 120-1, 120-2, . . . , and 120-n, a plurality of modulators 130-1, 130-2, . . . , and 130-n, and a plurality of antennas 140-1, 140-2, . . . , and 140-n. In FIG. 1, C1 to Cn denote spreading code sequences used in the transmitter.
As shown in FIG. 2, the conventional diversity receiver includes a plurality of antennas 210-1, 210-2, . . . , and 210-3, a plurality of demodulation devices 220, 230, and 240, a combiner 250, and a band pass filter 260. Each of the plurality of demodulation devices 220, 230, and 240 may include a demodulator 221, a plurality of first multipliers 222-1, 222-2, . . . , and 222-n, a plurality of accumulators 223-1, 223-2, . . . , and 223-n, a plurality of second multipliers 224-1, 224-2, . . . , and 224-n, a plurality of real part selectors 225-1, 225-2, . . . , 225-n, and a third multiplier 226. In FIG. 2, C1*, C2*, . . . , and Cn* denote complex conjugates of the spreading code sequences C1, C2, . . . , and Cn, which are used in the receiver, and α1,1*, α2,1*, . . . , and αn,1* denote complex conjugates of the path gains, which are used in the receiver.
Referring to FIG. 1, to increase the data transfer rate, conventional space-time coding must use a relatively dense modulation method.
Also, there has been proposed a method in which space-time block codes are used in the spreading code domain rather than the time domain. In this method, users are divided into groups, and each of the groups is assigned the same number of orthogonal spreading codes as the number of users in the group. A signal to be transmitted to each user in a group is encoded together with signals to be transmitted to the other users in the group. Although this method is disadvantageous in that each user must know all spreading codes used in its group, this method does not require additional spreading codes for each base station and can also achieve transmission diversity without time delay.
When the above method, which achieves transmission diversity using space-time block codes in the spreading code domain rather than in the time domain as described above, is extended by introducing a transmission matrix to the method, it is possible to much more generalize the method such that transmission diversity is achieved using Walsh codes, which are orthogonal spreading codes, without dividing the users into groups.
Specifically, one diversity method, which uses both space-time block codes and Walsh codes in downlink communication in a CDMA system, was disclosed in U.S. Pat. No. 6,515,978 (Feb. 4, 2003), entitled “Methods and apparatus for downlink diversity in CDMA using Walsh codes”.
Since this method uses the Walsh code domain rather than the time domain to achieve CDMA downlink diversity, it can be considered a method employing the conventional space-time block codes. Channels divided according to Walsh codes can be regarded as being identical to channels obtained by varying the time or frequency. The method of U.S. Pat. No. 6,515,978 has suggested an example of downlink diversity which can be achieved when orthogonal spreading codes such as Walsh codes are used, and also suggested general rules which make it possible to achieve downlink diversity. Although this patent has suggested a new method using the conventional space-time block codes, the method can merely achieve transmission diversity.
Also, there has been suggested a method that achieves transmission diversity in CDMA mobile communication using Space-Time Trellis Codes (STTC) rather than Space-Time Block Codes (STBC) and also increases the performance of the STTC by assigning one or more spreading codes to each user. This method is based on Orthogonal Plane Sequence Modulation (OPSM) that uses two or more orthogonal spreading codes rather than signal constellations.
This method is described in an article entitled “Space-time trellis-codes for high data rate wireless CDMA systems,” IEE Electronic Letters, Vol. 39, No. 6, P. 541 to 543, Mar. 20, 2003.
This prior art has proposed new Space-Time Trellis Codes (STTC) by increasing the number of spreading codes per user to L, and also proposed a method for improving the performance of the STTC. Also, this prior art showed that the conventional space-time codes are applicable to Orthogonal Plane Sequence Modulation (OPSM), and further disclosed a new application of conventional space-time codes to the OPSM. However, this prior art still does not change the conventional space-time code design.
This prior art has an advantage in that coding gain is achieved through spread signals using OPSM and system adaptability is also improved by diversifying the demodulation method which makes it possible to implement a single data transfer rate. However, this prior art merely applies spread codes in a different manner under the assumption that the spread codes are orthogonal to each other. Thus, an efficient method for designing space-time codes has not yet been proposed.